After my boyfriend stays the night, by pillows smell bad. Why?

He never smells bad, just the pillow he is laying on. And it only just started. I used to take him pillow from his house cause it was softer than mine, and it never smelt bad. now when he stays with me my pillows stink. kinda like dog... its weird and I know its not me because I wash them and they don't smell till he comes over again and he hates my feather pillow so I use it when he comes over and it doesn't smell. I haven't washed the feather one yet. how can I keep them smelling clean?